For property tax purposes, a Farm is defined as follows:

    (35 ILCS 200/1-60)

 Sec. 1-60. Farm. When used in connection with valuing land and buildings for an agricultural use, any property used solely for the growing and harvesting of crops; for the feeding, breeding and management of livestock; for dairying or for any other agricultural or horticultural use or combination thereof; including, but not limited to, hay, grain, fruit, truck or vegetable crops, floriculture, mushroom growing, plant or tree nurseries, orchards, forestry, sod farming and greenhouses; the keeping, raising and feeding of livestock or poultry, including dairying, poultry, swine, sheep, beef cattle, ponies or horses, fur farming, bees, fish and wildlife farming. The dwellings and parcels of property on which farm dwellings are immediately situated shall be assessed as a part of the farm. Improvements, other than farm dwellings, shall be assessed as a part of the farm and in addition to the farm dwellings when such buildings contribute in whole or in part to the operation of the farm. For purposes of this Code, "farm" does not include property which is primarily used for residential purposes even though some farm products may be grown or farm animals bred or fed on the property incidental to its primary use. The ongoing removal of oil, gas, coal or any other mineral from property used for farming shall not cause that property to not be considered as used solely for farming.
(Source: P.A. 86-1481; 87-877; 88-455.)

In order to be considered a farm, the property must have met this definition in the two prior years, as well as in the current year:

(35 ILCS 200/10-110)

Sec. 10-110. Farmland. The equalized assessed value of a farm, as defined in Section 1-60 and if used as a farm for the 2 preceding years, except tracts subject to assessment under Section 10-145, shall be determined as described in Sections 10-115 thr ough 10-140. To assure proper implementation of Sections 10-110 through 10-140, the Department may withhold non-farm multipliers for any county other than a county with more than 3,000,000 inhabitants that classifies property for tax purposes.
(Source: P.A. 92-301, eff. 1-1-02.)

The statutory process for farm assessment is as follows:

(35 ILCS 200/10-115)

Sec. 10-115. Department guidelines and valuations for farmland. The Department shall issue guidelines and recommendations for the valuation of farmland to achieve equitable assessment within and between counties.

The Director of Revenue shall appoint a five-person Farmland Assessment Technical Advisory Board, consisting of technical experts from the colleges or schools of agriculture of the State universities and State and federal agricultural agencies, to advise in and provide data and technical i nformation needed for implementation of this Section.

By May 1 of each year, the Department shall certify to each chief county assessment officer the following, calculated from data provided by the Farmland Technical Advisory Board, on a per acre basis by soil productivity index for harvested cropland, using moving averages for the most recent 5-year period for which data are available:

(a) gross income, estimated by using yields per acre as assigned to soil productivity indices, the crop mix for each soil productivity index as determined by the College of Agriculture of the University of Illinois and average prices received by farmers for principal crops as published by the Illinois Crop Reporting Service;
(b) production costs, other than land costs, provided by the College of Agriculture of the University of Illinois;
(c) net return to land, which shall be the difference between (a) and (b) above;
(d) a proposed agricultural economic value determined by dividing the net return to land by the moving average of the Federal Land Bank farmland mortgage interest rate as calculated by the Department;
(e) the equalized assessed value per acre of farmland for each soil productivity index, which shall be 33-1/3% of the agricultural economic value, or the percentage as provided under Section 17-5; but any increase or decrease in the equalized assessed value per acre by soil productivity index shall not exceed 10% from the immed iate preceding year's soil productivity index certified assessed value;
(f) a proposed average equalized assessed value per acre of cropland for each individual county, weighted by the distribution of soils by productivity index in the county; and
(g) a proposed average equalized assessed value per acre for all farmland in each county, weighted (i) to consider the proportions of all farmland acres in the county which are cropland, permanent pasture, and other farmland, and (ii) to reflect the valuations for those types of land and debasements for slope and erosion as required by Section 10-125.
(Source: P.A. 91-357, eff. 7-29-99.)

(35 ILCS 200/10-125)

Sec. 10-125. Assessment level by type of farmland. Cropland, permanent pasture and other farmland s hall be defined according to U.S. Census Bureau definitions in use during that assessment year and assessed in the following way:

(a) Cropland shall be assessed in accordance with the equalized assessed value of its soil productivity index as certified by the Department and shall be debased to take into account factors including, but not limited to, slope, drainage, ponding, flooding, and field size and shape.
(b) Permanent pasture shall be assessed at 1/3 of its debased productivity index equalized assessed value as cropland.
(c) Other farmland shall be assessed at 1/6 of its debased productivity index equalized assessed value as cropland.
(d) Wasteland shall be assessed on its contributory value to the farmland parcel.

In no case shall the equalized assessed value of permanent pasture be below 1/3, nor the equalized assessed value of other farmland, except wasteland, be below 1/6, of the equalized assessed value per acre of cropland of the lowest productivity index certified under Section 10-115.
(Source: P.A. 86-954; 88-455.)

(35 ILCS 200/10-135)

Sec. 10-135. Farmland not subject to equalization. The assessed valuation of farmland assessed under Sections 10-110 through 10-130 shall not be subject to equalization by means of State equalization factors. Equalization factors applied by a chief cou nty assessment officer or a Board of Review under Sections 9-205 and 16-60 shall be applied to assessments of farmland only to achieve assessments as required by Sections 10-110 through 10-130.
(Source: P.A. 92-301, eff. 1-1-02.)

(35 ILCS 200/10-140)

Sec. 10-140. Other improvements. Improvements other than the dwelling, appurtenant structures and site, including, but not limited to, roadside stands and buildings used for storing and protecting farm machinery and equipment, for housing livestoc k or poultry, or for storing feed, grain or any substance that contributes to or is a product of the farm, shall have an equalized assessed value of 33 1/3% of their value, based upon the current use of those buildings and their contribution to the productivity of the farm. (Source: P.A. 86-954; 88-455.)

(35 ILCS 200/10-145)

Sec. 10-145. Farm dwellings. Each farm dwelling and appurtenant structures and the tract upon which they are immediately situated shall be assessed by the local assessing officials at 33 1/3% of fair cash value except that in counties that classify property for purposes of taxation in accordance with Section 4 of Article IX of the Constitution they shall be assessed at the percentage of fair cash value as required by county ordinance. That assessment shall be subject to equalization by the Department under Sections 17-5 through 17-30.
(Source: P.A. 82-554; 88-455)